Video circuits are used in conjunction with video display systems in which a raster scan video display comprises a plurality of raster scan lines. Typically such a display comprise a plurality of successively displayed frames wherein each frame comprises a pair of successive fields. A frame in the United States comprises 525 scan lines and a field comprises 262.5 scan lines. The fields are each defined by a vertical synchronization signal and each scan line is defined by a horizontal synchronization signal.
Often, problems arise in the video generating and display circuits. Some of these problems appear as undesired artifacts in a portion of the visual display. Others of these problems may appear which undesirably affect the entire display. In addressing these problems the service technician typically employs an oscilloscope to display the circuit waveforms at selected points in the circuit in an attempt to isolate the cause of the displayed problem which appears on the video monitor, receiver and so on. To do this, the test points of the circuit provide vertical and horizontal components of the oscilloscope display. A trigger signal is also supplied the oscilloscope as part of the oscilloscope operation.
The trigger signal is intended to permit display on the oscilloscope display a given portion of the waveform to be analyzed. The problem with displaying a video signal on an oscilloscope, as recognized by the present inventor, is the lack of a reliable trigger signal source for isolating a given portion of a video signal. Typically the oscilloscope displays too much of the video signal so as to be substantially of little value in isolating a particular cause of a video display problem. Trigger signal generating devices are available but these usually have fixed delay relative to a reference, for example, the vertical sync signal. This fixed delay results in the trigger signal being based on an arbitrary portion of a video signal which may vary in amplitude and therefore cause loss of the trigger signal or, at best, cause jitter in the display of the oscilloscope. Further, more importantly, the defective portion of a video signal, i.e., the portion exhibiting an artifact, caused by a defective video circuit portion may or may not be included in the oscilloscope display initiated by that trigger signal generating device.